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GATE 2026 BT Q.22: solution and answer

GATE 2026 · BTQ.22 · 1 mark · MCQ

The RNA primer synthesized during bacterial DNA replication is removed by ________.

  1. (A)

    DNA gyrase

  2. (B)

    DNA Polymerase I

  3. (C)

    Primase

  4. (D)

    DNA Polymerase III

See the question in the paper
GATE 2026 BT question 22Tap to open full size

Answer and solution

ANSWER

The correct answer is (B) DNA Polymerase I, which removes the RNA primer via its 5'→3' exonuclease activity during bacterial DNA replication.

Official answer: BSource: Official answer key, GATE 2026 BT (IIT Guwahati)

  1. DNA gyrase (A) is a type II topoisomerase that relieves supercoiling ahead of the replication fork; it plays no role in primer excision, so A is incorrect.

  2. Primase (C) is the enzyme that synthesizes the RNA primers in the first place; it does not remove them, so C is incorrect.

  3. DNA Polymerase III (D) is the main bacterial replicative polymerase, extending primers and synthesizing the bulk of leading- and lagging-strand DNA, but it possesses only 3'→5' proofreading exonuclease activity and lacks 5'→3' exonuclease activity, so it cannot excise primers — D is incorrect.

  4. DNA Polymerase I (B) uniquely combines 5'→3' polymerase activity with 5'→3' exonuclease activity, letting it simultaneously excise the RNA primer ahead of it and fill the resulting gap with DNA — this is nick translation, the standard mechanism for primer removal during Okazaki fragment maturation.

  5. The nick left after Pol I finishes is sealed by DNA ligase (not an option here), completing lagging-strand maturation.

  6. Hence the correct answer is (B) DNA Polymerase I.

KEY IDEA

In bacteria, DNA Polymerase I is the only replicative polymerase with 5'→3' exonuclease activity, letting it simultaneously excise RNA primers and fill the gap with DNA (nick translation); DNA ligase then seals the remaining nick.

WATCH OUT FOR

Confusing DNA Polymerase III (the main chain-extending replicative enzyme) with DNA Polymerase I (the primer-removal/gap-filling enzyme) — students often forget Pol III lacks 5'→3' exonuclease activity and cannot excise RNA primers.

From the GATE 2026 BT paper. Every question of this paper · Download the full paper · All 21 Gene expression questions, with notes

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